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Current Limits for UniPolar/Bipolar Steppers

I have recently purchased a Unipolar/Bipolar, 200 Steps/Rev, 42×48mm, 4V, 1.2 A/Phase (#1200 (SY42STH47-1206A).
The specification sheet for the device says the resistance per phase is 3.3 Ohms however when measuring across the green & black and blue & red I get a resistance of 7.2Ohms. When measuring between Green & Yellow I get around 3.6 which appears to be more like the specified value (within ±10%).

Is the specification for resistance per coil with respect to a Unipolar set-up? if so does this mean that the current limit per coil is actually half the specified 1.2A and is actually 600mA and the phase resistance is more like 6.6Ohm if I am utilising a Bipolar configuration?


For product #1200 (SY42STH47-1206A), the rated current, resistance per phase, and other specifications in the datasheet are given for how the motor would be used in a unipolar configuration. In other words, the specs apply from one end of each phase to the center-tap wire.

If you use the motor as a bipolar stepper motor (as you would have to do if you are using any of our stepper motor drivers or Tic stepper motor controllers), then the characteristics will change. As you observed with your measurements, the resistance doubles. For the new rated current you should multiply by a factor of 0.707, and for the new rated voltage you should multiply by a factor of 1.414. Here is a blog post with more detailed information about the differences between unipolar and bipolar stepper motor operation.

- Patrick